Tin, lead and tin-lead alloy deposits have been used for many years in the electronics industry. These deposits are used, in particular, in the manufacture of printed circuit boards, electrical contacts and connectors, semiconductors, electrical conduit, and other related parts. These alloy deposits are used to improve solderability and to form etching resistant films on electrical and electronic parts. Plating of electronics is achieved through the use of electroplating solutions containing tin, lead, or tin-lead alloys. These baths are widely employed to permit high speed uniform metal plating of tin, lead or tin-lead alloy.
Traditional electroplating baths have utilized fluoborates in solution with the two metals with an excess of fluoboric and boric acid. The fluoborate-based baths have been successful in obtaining electrical and electronic components with smooth matte deposits. However, these baths are problematic since the fluoborate baths are extremely corrosive to equipment and highly toxic. Further, fluoborates are difficult to remove from the waste water that follows the plating operation.
More recent developments in electroplating baths use alkane or alkanolsulfonic acids containing one to five carbon atoms which are capable of forming soluble complex salts with divalent tin and lead. See for example U.S. Pat. No. 4,459,185, herein incorporated by reference. The alkane or alkanolsulfonic acids provide tin, lead and tin-lead alloy plating baths capable of giving uniform and dense deposits on substrate surfaces at high speed, without employing the fluoborates that can cause environmental pollution problems. U.S. Pat. No. 4,459,185, herein incorporated by reference, further describes the addition of one or more specified cationic, amphoteric and/or nonionic surfactants and one or more leveling agents to an alkane- or alkanolsulfonic acid plating bath and a divalent tin salt, divalent lead salt, or a tin/lead mixture.
Modern tin and/or lead alloy plating baths require tin and lead to exist in the divalent state. Problems commonly associated with tin and tin-lead alloy plating baths include factors present during the electroplating process which can cause the oxidation of tin from the divalent state to the tetravalent state. In the high speed electroplating process, for example, the rapid pumping action and solution movement common to high speed plating machines causes air to be mixed with the solution, thereby promoting such oxidation. This oxidation is accelerated due to the elevated temperature of operation which is commonly used in high speed plating installations.
When divalent tin becomes tetravalent, it tends to precipitate in the bath as stannic oxide, thereby forming a sludge with a corresponding loss of available metal for electrodeposition. This sludge reduces the efficiency of the bath and causes operating problems because of its tendency to clog the jets and sprayers of the agitation system. This in turn results in frequent and costly production shut downs for clean up and removal. It is therefore very important to use plating baths which will substantially limit the formation of tetravalent tin under the particular electroplating conditions used.
The prior art discloses a method of reducing tetravalent tin and stannic oxide sludge formation in high speed tin or tin-lead electroplating solutions using an antioxidant, such as pyrocatechol, resorcinol, and hydroquinone, and a soluble alkyl or alkylol sulfonic acid in an amount sufficient to provide a pH of less than 3 to the solution (see U.S. Pat. No. 5,066,367, herein incorporated by reference). However, use of these well-known antioxidants have still resulted in the formation of large quantities of sludge during the electroplating process.
According to U.S. Pat. No. 4,681,670, the pH of methane sulfonic acid baths may be raised to a preferred range of between 3 and 4 using alkali hydroxides, such as sodium and potassium hydroxide. Raising pH by sodium hydroxide, however, is problematic for a number of reasons. First, the reaction of sodium hydroxide with the alkane- or alkyl sulfonic acid bath at increased pH oxidizes the tin and/or lead which results in the metal precipitating out of solution to form a sludge. Secondly, the reaction between alkane- and alkyl sulfonic acid baths and alkali hydroxides proceed very quickly and generate large amounts of heat. The speed of these reactions also adds to the formation of metal precipitate within the electroplating bath. Also, the alkali hydroxides are highly toxic if inhaled and are strong tissue (eye, skin, mucous membrane) irritants. Thus, they are extremely difficult to work with.
There is thus a need in the art for an alkyl and alkylol sulfonic acid plating bath which prevents the formation of sludge due to oxidation within the solution and which is safe to work with.
It is therefore a primary object of the present invention to provide a tin and/or lead electroplating solution which reduces the amount of oxidation in the solution during processing, thus limiting the amount of sludge formation.
It is another object of the present invention to provide a tin and/or lead electroplating solution which utilizes slower reaction times and generates less heat than prior art reaction solutions.
It is yet another object of the present invention to provide a tin and/or lead electroplating solution which is safe to use and economical to make.
The method of accomplishing these and other objects will become apparent from the following description of the invention.